1. Field of the Invention
The present invention relates to a weight adjustment device suitable for use in a seat suspension for a vehicle such as an automobile.
2. Description of the Prior Art
Conventionally, many of seats for an automobile or the like are provided with a so-called seat suspension device which is adapted to support the seat resiliently so as to absorb the vibrations thereof, and, recently, a new type of seat suspension is proposed which is provided with a weight adjustment device for obtaining a predetermined seat support height at all times even when occupants of different weights are seated.
In FIG. 5, there is illustrated an example of the conventional seat suspension weight adjustment devices of this type. In FIG. 5, reference numeral (1) designates an upper frame and (2) represents a lower frame. The upper frame (1) and lower frame (2) are connected through two pairs of X-shaped links (3)(3) to each other in such a manner that they can be oscillated vertically.
Each of the X-shaped links (3), as is well known, comprises two link bodies (4) and (5) both of which are pivotally supported substantially at their central portions by a shaft (6) such that they are free to rotate. The rear-side (or front-side) upper end (5a) of each of the X-shaped links (3) is mounted to the upper frame (1) by another shaft (7) in a freely rotatable manner, while the lower end (4b) thereof is mounted to the lower frame (2) by another shaft (8) in a freely rotatable manner. Also, the front-side (or rear-side) lower end (5b) of each X-shaped link (3) is attached to the lower frame (2) by a roller (9) or the like such that it is free to slide, that is, it can be moved back and forth. The front-side upper ends (4a) of the respective X-shaped links (3) are connected with each other by means of a frame (10) extended therebetween, which serves to support the front portion of the upper frame (1) from below.
The rear-side upper ends (5a) of the two X-shaped links (3) are also connected with each other by means of another frame (11) to which the respective first ends of a pair of springs are secured.
A bell crank (14) is rotatably mounted via a pivot shaft (13) to the upper frame (1), with one end thereof bening engaged with an adjust shaft (15) extending laterally and the other end thereof being connected through a shaft (17) to one end of a tension frame (16) in a rotatable manner. This tension frame (16) is connected to an equalizer frame (18) in such a manner that it is rotatable about a shaft (19), and the second ends of a pair of springs (12) mentioned above are respectively secured to the two ends of the equalizer frame (18).
The above-mentioned adjust shaft (15), in this embodiment, is formed as a rotary shaft which is formed in a portion thereof with a screw slot (20). A nut (21) is threadedly engaged with this slot (20) and the above-mentioned one end of the bell crank (14) is engaged with this nut (21).
Also, substantially centrally of such conventional weight adjustment device, there is provided a shock absorber (22) with one end thereof being rotatably mounted to the rear-side side of the lower frame (2) and the other end thereof rotatably mounted to the frame (10).
In operation, if the adjust shaft (15) is rotatively operated to move the nut (21) back and forth, then the tensile force of the tension spring (12) can be varied through the bell crank (14) and tension frame (16), so that, even when the occupants of different weights are seated, the upper frame (1) can be adjusted such that it is always supported in a predetermined height level.
In the above-mentioned prior art arrangement, however, partly because a sufficient space is not available between a pair of springs (12) and partly because a screw mechanism including a screw and a nut block is employed as the adjustment mechanism thereof, when adjusting, for example, between 50 kg and 100 kg, the adjust shaft (15) must be rotated (manually) a considerable number of times, resulting in a very poor operationability. Also, the operation forces required for such adjustment become greater as the adjustment approaches toward 100 kg, and, actually, while the door of the car is closed, the adjustment operation is impossible.
Further, since the above adjustment mechanism is disposed on the upper frame (1) side, when a cushion body of the seat proper is directly mounted onto the upper surface of the upper frame (1), the cushion body interferes with the adjustment mechanism to produce a strange feeling at the sitting time. To avoid this, after another cushion frame is mounted onto the upper surface of the upper frame (1), the cushion body is then placed on the cushion frame. As a result of this, the prior art weight adjustment device is increased in weight and thickness and thus it is difficult to apply such prior art device to an ordinary automobile or the like which has a limited space.